I Have A 2002 ML320 W/96000 Miles. I've Only Had It For 2 Weeks. I Replaced The Battery 1 Week Ago And Then 2 Weeks To The Day The Check Engine Light Came On And It Started To Idle Rough. I Unplugged Battery And Reconnected The Light Went Off And It Ran Great. Later That Night Light Came On Again. Changed Spark Plugs (which Hadn't Been Changed In A Long While) And Added Fuel System Cleaner With Higher Octane Gas. Started To Get Better, Next Day Worse. Took To Autozone And Got Codes:p0303-misfire Cylinder 3, 90203-malfunction Exists In Electrical Circuit For Cylinder 3 Fuel Injector. What Should I Do Next Any Suggestions Please?!